The hidden assumption is that the difference is fundamental rather than circunstancial — maybe a sufficiently powerful computer running a sufficiently sophisticated program can be conscious.
Maybe it can. People presenting the hard problem of consciousness do not usually exclude such a thing. The question is, what causes human qualia? The actual experience of stimulus, rather than merely being a system reacting to them in a certain way.
The only way I can understand someone rejecting this assumption is if they somehow do not actually experience qualia.